The use of pulse code modulation (PCM) techniques in the context of digital communication systems is generally well known. For example, PCM encoding schemes have been utilized in connection with the public switched telephone network (PSTN) for many years to enable the encoding of voiceband signals. An internationally standardized PCM encoding protocol is set forth in ITU-T Recommendation G.711 (International Telecommunication Union, 1993). Recommendation G.711 contains the μ-law encoding scheme, which is utilized in the United States (and other countries), and the A-law encoding scheme, which is utilized in European and other countries. The entire content of ITU-T Recommendation G.711 is incorporated herein by reference.
PCM modems, such as 56 kbps modem systems compliant with ITU-T Recommendation V.90 (International Telecommunication Union, 1998), generally rely on the transmission and receipt of codewords or levels utilized by the μ-law or A-law encoding schemes (the entire content of ITU-T Recommendation V.90 is incorporated herein by reference). Due to the use of different standardized encoding schemes, a given end-to-end communication channel may be associated with a transmit modem located in one country and a receive modem located in a different country. Accordingly, the transmit-receive communication channel may be designated as: (1) μ-law to μ-law; (2) μ-law to A-law; (3) A-law to μ-law; (4) A-law to A-law; or (5) a combination of thereof (e.g., where a number of different conversions are utilized).
Section 3.4 of Recommendation G.711 States that:                Digital paths between countries which have adopted different encoding laws should carry signals encoded in accordance with the A-law. Where both countries have adopted the same law, that law should be used on digital paths between them. Any necessary conversion will be done by the countries using the μ-law.Consequently, in some cross-country situations, the transmit modem codec may follow a different encoding law than the central office codec associated with the receive modem. If the actual μ-law or A-law encoding levels are to be utilized by the modem system, then the central office codec type must be known to or identified by the receive or transmit modem. If the central office codec type is known or identified, then the receive modem can match its slicer levels with the particular encoding scheme to reduce the quantization noise inherent in the system. If the central office codec is known to the transmit modem, e.g., the server modem in a V.90 system, then it can use such knowledge to better adjust its echo canceler.        
Although a transmit modem may transmit an identification signal (indicating its encoding law) to the receive modem, the encoding type of the central office cannot be easily ascertained from the identification signal. Accordingly, current receive modems may simply trust the identification signal and assume that the central office codec type matches the transmit modem codec type. In other words, the receive modem may not make an independent assessment of the central office codec type.